I find that to work effectively on large Word documents you really need to understand the underlying object model. Everything is an object, and the objects fit together in reasonably consistent ways. But when you don't understand the model then it seems like things happen randomly to the document format for no good reason.

It also helps to turn on the display option "Show all formatting marks". That way you can kind of see the objects and boundaries. Even though you can't directly manipulate those markers like tags in TeX or something they still give helpful clues as to what's happening.

(I'm not trying to defend the usability problems in Word, just pointing out some ways to live with them.)